Every Tuesday night (except the first one of each month), the Paragon restaurant holds a jam session, hosted by singer Billy Brandt. Although primarily a vocal jam backed by guitar and keyboard, instrumental musicians, including horns, keyboard, and bass players, are welcome to sit in. Billy has a sign-up sheet, but it is casual.
Each week, the event is kicked off by a featured singer in the first half hour. After that, Billy opens it up to other musicians.
The venue is a bar, so you must be 21 or older to participate.
More about the Paragon: http://www.paragonseattle.com/
More about Billy Brandt: http://billybrandtmusic.com/